Filter Factory (free)

... is a Photoshop-compatible filter plugin
that enables you to create your own filter effects. To achieve effects
with Filter Factory you have to type C like formulas for processing
the Red, Green, Blue and Alpha channel of 24bit images. To get a deep
insight please read the Filter
Factory Programming Guide.

Filter Factory is available for Windows and
MacOS 8/9. As it hasn't been updated since 1995 it can't be used with
Mac OS X. The Filter Factory plugin is included on the Photoshop 3,
4, 5 or 6 CD-ROMs (for Macintosh and Windows) as well as on the Photoshop
7 and CS for Windows CD-ROM. Filter Factory for Windows needs Msvcrt10.dll
to work. There were two special version of Filter Factory included with
Adobe Premiere 4 and 5 called Filter Factory and Transition Factory.
Unfortunately Adobe doesn't deliver them anymore since Version 6 of
Adobe Premiere. The Filter Factory versions of Premiere 5 works in Premiere
6, but not in Premiere Pro.

Plugin
Commander Pro lets you easily convert Filter Factory plugins to Macintosh
versions as well as Premiere versions. You can also use it to edit and
preview your Filter Factory plugins.

Who want to make their filters available for MacOS
8 and 9 without much additional work

Who want to make their filters available for Premiere
without much additional work

Filter Foundry ($5
donationware)

Filter Foundry is a compatible
replacement for Adobe Filter Factory. It may contains a few inconsistencies
with Filter Factory that should be reported to the author, so that
they can be fixed in the next release. It works with Photoshop 3.0-7.0
on Power Mac (OS X, Classic, OS 9 and earlier), 68K Mac and Windows.
Unlike the other tools mentioned here, the filter code is interpreted
and not compiled to machine code. However, you won't notice a speed
difference if you use an up-to-date processor. Improvements on Filter
Factory are a larger preview (which is correct at all zoom rates),
relieved expression length limits and OS X support.

Filter Foundry is recommended for people:

Who don't want to invest much money

Who have little programming knowledge

Who want to make their filters available for 68K
Mac and PowerMac (MacOS 8, 9 and X) without much additional
work

... is a Photoshop-compatible
plugin for Windows. It is similar to Filter Factory for creating your
own filter effects. FilterFormula (short: FiFo) supports Filter Factory
source code, but has has some advantages over Filter Factory:

1. It doesn't crash when a filter with the cnv command
is used on larger images.

2. It doesn't need Msvcrt10.dll or Plugin.dll.
3. It can handle (in many cases) images that don't fit into main memory.
4. It creates stand alone plugins that aren't decompilable.
5. The filter code is entered in only one text box, instead of 3 or
4 as in Filter Factory.
6. It let's you include multiple filters in one plugin (from Version
1.1 on).
7. It has a zoomable and pannable preview box.
8. The range of each slider can de defined by the user.
9. It has 6 color selector buttons and 3 check boxes.
10. The filter code can contain for & while loops and
if ... else statements.
11. There are 65535 storage cells, instead of 256 as in Filter Factory.
12. It has a extended programming language with more commands.
13. The source code limit is 16K (in Filter Factory 4K) and the compiled
code limit is 8K (in Filter Factory something around 1-2K).
14. It can display Help files for standalone filters (Help button).

Unfortunately the latest Filter Formula release is from
1999 and there are no signs that it will be developed further.

Filter Formula is recommended
for people:

Who are familiar with Filter Factory or have a
bit of programming knowledge

FilterMeister
(FM) is a plugin for Adobe Photoshop and plugin-compatible applications.
Although it has a "Beta" added to version name for 10 years now,
you should not hesitate to use it. It has been toroughly tested
and is still developed further. It lets you create your own Photoshop-compatible
plugin filters using a powerful C-like programming language. You
also have complete control over the design and layout of the user
interface for the filters you create. Here's a feature list:

The FilterMeister programming
language:

Is backward compatible with the Filter Factory programming
language. You can import your old Filter Factory code and convert
it to an FM filter with the click of a few buttons. Filter Formula
code can also easily adapted to FilterMeister. For example the
ForEveryPixel handler work similar to Filter
Formula.

Implements a large subset of the C programming language.

Has the complete set of C-language control constructs (such
as for-loops, while-loops, and switch statements), allowing you
to code advanced algorithms that can't be expressed in Filter
Factory.

Supports all the basic C-language data types, including floating-point
data types. You are no longer restricted to integer arithmetic
as in Filter Factory.

Contains hundreds of built-in functions and variables, giving
you fast and powerful access to all the features needed to access
and manipulate images.

Gives you instant access to most of the standard C run-time
routines, including memory allocation, string manipulation, math
functions, and file input/output.

The FilterMeister user interface:

Is fully programmable, giving you complete control over the
appearance and function of the filters you create.

Supports an ever-growing list of user controls:

sliders (scrollbars and trackbars)

pushbuttons, radio buttons, and checkboxes

group boxes, frames, and rectangles

bitmaps, icons, and metafiles

edit boxes and static text boxes

list boxes and dropdown combo boxes

progress bars

zoomable and pannable proxy image previews

owner-drawn controls for unlimited special effects

Lets you create tool tips for all user controls.

Allows you to specify the dialog background as a solid color,
a gradient, or a graphic image.

Gives you control over the size and configuration of the dialog
box, and even lets you specify a nonrectangular shape for the
dialog.

The plugin dialog can be made resizable.

The FilterMeister compiler:

Emits native x86 machine code for fast execution.

Generates in-line code for many of the basic Filter Factory
functions.

Imposes no practical limit on source code and object code size,
allowing you to write very complex filter algorithms.

Generates code that links only to the standard Win95/98/NT dynamic
link libraries, so no special DLL files are required.

... is a plugin development solution for
Borland Delphi 5, 6, 7 and for Borland C++ Builder 6. Basically
the software consists of two main parts: the plug-in development
wizard and the filter host component.

Some of the features include:

Contains pascal conversion of parts of the Photoshop
API

Support for several different kinds of plug-in modules:
Filter, Import, Export, Color picker, Format

IDE expert for managing PiPL (Plug-in Property List)
resources

Lets you add, remove or modify plug-in specific properties

Filter host component for executing plug-in filters

Create your own Photoshop host applications within minutes

Lets you load all plug-ins in a directory tree

Delphi conversion of the Wintab API

Support for tablet devices within plug-ins

Lets you develop filter plug-in modules with ease

Use a patched version of the Graphics32 controls for
displaying intermediate filter output to the screen

... contains instructions and source codes
for developing Photoshop-compatible plugins. This is the hardest,
but also most flexible way to create your own filter plugins. In
recent years you had to become an ASN member for $195/year in order
to obtain the SDK, but now you only need to register
for a free Adobe ID to get the SDK. Previous version of the
SDK can be found on the Photshop 3/4/5/6 CD-ROMs. The older Photoshop
SDKs (3/4/5/6) can also be used to create plugins for Photoshop
7, CS, CS2 and CS3. For making your plugins compatible to as many
other graphics applications you should better use an older version
of the Photoshop SDK and test them with as many applications as
possible. In addition to Photoshop you should at least test your
plugins with Paint Shop Pro.